DIAS, L. C. S.; GLASSER, C. M.; MARÇAL Jr., O. & BONESSO, P. I. P. Epidemiology of Schistosomiasis Mansoni in a Low Endemic Area. Cad. Saúde Públ., Rio de Janeiro, 10 (supplement 2): [254][255][256][257][258][259][260] 1994. We discuss the epidemiological patterns of schistosomiasis mansoni in areas with low transmission in Brazil. We define as areas of low endemicity those where the prevalence is less than 10%, the number Schistosoma mansoni eggs per gram of feces (epg) is less than 96, and carriers are asymptomatic. Data are from the county of Pedro de Toledo in the Ribeira Valley (São Paulo State) and were collected randomly according to the aggregate pattern of S. mansoni within the hosts. We suggest the replacement of parasitological methods by more sensitive and specific serological techniques. The main risk factor for infection is type of leisure activity. Infection is more frequent in the 10-14, 15-19, and 20-24-year age brackets. Geometric mean epg is 58.5. Intensity of infections correlates well (rs = 0.745) with prevalence. The highest index of potential contamination is in the 5-20-year age bracket (57.6%). Autochthonous cases show close association with Biomphalaria tenagophila, which has a low infection rate (2%). Prevalence, incidence, and intensity of infection patterns are similar to those of moderate and high endemic areas. Social and cultural aspects must be studied in order to obtain a global epidemiological view of schistosomiasis. Key words: Schistosomiasis; Schistosoma mansoni; Low Endemicity; Epidemiology INTRODUÇÃOA epidemiologia da esquistossomose em suas linhas gerais é bem conhecida. Ela não é, necessariamente, uniforme dentro de um país endêmico e é quaseimpossível de ser comparada entre países. Sua epidemiologia é tão variada quanto a ecologia humana e o ambiente no qual a esquistossomose ocorre. Os parâmetros epidemiológicos, como prevalência, incidência, intensidade de infecção e morbidade, variam, amplamente, mesmo dentro de uma região. Essa diversidade pode ser explicada, principalmente, pela ecologia humana e o ambiente (Doumenge et al., 1987) É tarefa difícil propor parâmetros para avaliar o grau de endemicidade da esquistossomose mansônica devido a seu caráter focal e sua ampla diversidade. Todavia, pode tentar uma classificação. Uma área seria considerada de alta endemicidade quando houvesse altas prevalência e intensidade de infecção, geralmente, em crianças entre 5 e 15 anos de idade e formas crônicas em adultos. Nas áreas de moderada ou baixa endemicidade, a distribuição geográfica dos portadores e da morbidade severa estaria bem localizada, em focos nitida-
For a period of 2 years, five follow-up measures of prevalence and incidence rates were estimated in a prospective study of S. mansoni infection in a group of schoolchildren who were living in a rural area of the Municipality of Itariri (São Paulo, Brazil), where schistosomiasis is transmitted by Biomphalaria tenagophila. Infection was determined by the examination of three Kato-Katz stool slides, and the parasitological findings were analyzed in comparison to serological data. In the five surveys, carried out at 6-month intervals (March-April and September-October), the prevalences were, respectively, 8.6, 6.8, 9.9, 5.8 and 17.2% by the Kato-Katz, and 56.5, 52.6, 60.8, 53.5 and 70.1% by the immunofluorescence test (IFT). Geometric mean egg counts were low: 57.8, 33.0, 35.6, 47.3 and 40.9 eggs per gram of feces, respectively. Of the total of 299 schoolchildren, who submitted five blood samples at 6-month intervals, one for each survey, 40% were IFT-positive throughout the study, and 22% were IFT-negative in all five surveys. Seroconversion from IFT negative to positive, indicating newly acquired S. mansoni infection, was observed more frequently in surveys carried out during March-April (after Summer holidays), than during September-October. Seasonal trends were not statistically significant for detection of S. mansoni eggs in stool. The results indicate that the use of IgM-IFT is superior to parasitological methods for detection of incidence of S. mansoni infection in areas with low worm burden.
SUMMARYDue to the scarce information about the epidemiological features of schistoso¬ miasis in which the vector is Biomphalaria tenagophila, an investigation was carried in Pedro de Toledo in 1980 where such peculiarity is observed. Stool examinations (Kato Katz method) were performed in 4,741 individuals (22.8% positive to Schistosoma mansoni eggs) of this 583 had previously received chemoterapy and 4,158 remainders, untreated. The schistosomiasis prevalence in those two groups where respectively 31.7% and 21.6% . Epidemiological investigation showed that 83.6% were autochthonous cases from the studied area: the autochthonous prevalence rate, and the intensity of infection in the untreated autochthonous cases were higher in males than in females; the intensity in the latter untreated group was low, 58.5 eggs/g feces {geometric mean). Moreover, according to the age groups the intensity of infections correlated well (r s = 0.745) with the prevalence rates. Schistosomiasis was veri¬ fied to occur mostly during the leisure time and by the use of water streams for housework in rural zone. Only 0.4% out of 1,137 snails was positive for S. mansoni cercariae, apparently unchanged from the 1978 study when the human prevalence was 12.0%. The studied area presented differences and similarities in relation to the other Brazilian areas were the main intermediate host is B. glabrata.
In population surveys in which the Schistosoma mansoni intensity of infection is low, or in localities where the schistosomiasis control program had success, the parasitologic methods lack in sensitivity. Despite of some limitations, the immunological methods are useful to provide valuable information in such field conditions. Thus, the prevalence of schistosomiasis in untreated population can be determined by the detection of IgG or IgM antibodies, as well as the incidence by the IgA antibodies, employing mainly immunofluorescence (IF) and immunoenzymatic (ELISA), and in some extent hemagglutination (HA) or even skin test. The true prevalence and incidence of schistosomiasis can be estimated using a probabilistic model equation, since knowing before-hand the sensitivity and specificity of employed test. The sensitivity and the specificity of serologic test become higher in low aged group, under 14. The geometric mean IF titers also gives a positive correlation with the intensity of infection. Presently, there are need of serologic tests which are economic and practical in seroepidemiologic inquiries, requiring no specialized personnel to collect population blood or serum samples, and also easily interpret the test results. The reagents for such tests are desired to be stable and reproducible. Moreover, it is expected that the tests can distinguish an active infection.
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